Current-limiting fusable device with fusable elements of stepped or graduated cross-sectional areas and having dents

ABSTRACT

A current-limiting fuse has one or more electrically parallel fusible elements, each of which has a stepped, or graduated cross-sectional diameter. In addition, each of the fusible elements has dents provided along its axial length. Preferably, a plurality of such fusible elements are disposed in parallel within a tubular fuse holder filled with an arc-extinguishing granular material, such as silica sand.

United States Patent 1111 3,569,889

[72] Inventors Yukio Iwasaki [56] References Cited Amagasaki; UNITED STATES PATENTS Takamuka 3,413,586 11/1968 Salzer 337/29s x [21] Appl. No. 768,406

3,287,526 11/1966 Brandt 337/295 [22] Filed Oct. 17,1968 3 2 51,968 5/1966 Cameron.... 337/158(X) [45] Patented 1971 2 157 907 5/1939 1. h 337/159 731 Assignee Mitsubishi Denki Kabushiki Kaisha 1 1 445,902 4/1936 Great Britain 337/290 Primary ExaminerG. Harris 4 CURRE L [TING FUSABLE DEVI E WITH Assistant Examiner-Dewitt M. Morgan [5 1 Attorneys-A. T. Stratton, Mcl-lale and Cl'Ollt GRADUATED CROSS-SECTIONAL AREAS AND gg ABSTRACT: A current-limiting fuse has one or more electrirawmg cally parallel fusible elements, each of which has a stepped, or [52] US. Cl 337/159, graduated cross-sectional diameter. In addition, each of the 337/295 fusible elements has dents provided along its axial length. [51] Int. Cl H0lh 85/08 Preferably, a plurality of such fusible elements are disposed in [50] Field of Search 337/158, parallel within a tubular fuse holder filled with an arc-extinguishing granular material, such as silica sand.

CURRENT-LIMITING FUSABLE DEVICE WITH FUSADILE ELEMENTS OF STEPPED OR GRADUATED (IROSS-SECTIONAL AREAS AND HAVING DENTS I CROSS-REFERENCES TO RELATED APPLICATIONS Applicants are not aware of .any related applications pertinent to the present invention.

BACKGROUND OF THE INVENTION The invention relates to fuses, and more particularly to power-fuse units of the current-limiting type having a plurality of fusible elements operating in parallel-current interrupting relation.

Such power-fuse units are extensively used in lieu of the relatively more expensive and larger size circuit breakers for protecting relatively high-voltage power circuits upon the occurrence of faults, particularly short circuits. In such short circuit protective service, the multielement current-limiting type of fuse has the outstanding advantage over circuit breakers of initially limiting the excess current to a value, which is only a fraction of that which the power circuit is otherwise capable of supplying.

. To obtain such current-limiting action, the multiple-fuse elements ordinarily are in the form of relatively long fine wires of a readily vaporizable metal, such as silver or zinc, and embedded in intimate heat exchangerelation throughout their active length in an inert, granular refractory arc-extinguishing material of high-dielectric strength, such as sand, or relatively finely divided quartz. The embedded elements are enclosed in an insulating fuse casing, or housing having terminals mounted thereon for connecting the fusible elements in parallel-current interrupting relation.

Upon subjection of such a fuse unit to fault currents of a high-voltage circuit, such as produced by a short circuit, the spaced-apart fusible elements, if all conditions are equal, will each attain fusing temperature and dissipate or vaporize throughout a fusible section of considerable length, whereupon parallel arcing will occur, and the metal vapors readily expand. The force of such expansion will throw the metal vapors out of the arc paths and into the spaces between the granules of the inert filler material separating the fusible elements. As a result, these vapors rapidly condense upon the surface of the relatively cool granules, and no longer are available for current conduction so that each arc is confined to the elongated and restricted path previously occupied by the corresponding fusible element. The intimate heat-exchange relation between the spaced-apart parallel elongated hot arc columns and the relatively cool granules will, if all conditions are equal, cause an exceedingly rapid transfer of heat from each arc column to the adjacent granules, so that the are energy is quickly dissipated by this heat transfer with practically negligible not pressure on the fusing housing.

As a result of the fusing of the parallel elements in the manner outlined above, there will be, in effect, suddenly inserted into the path of the current at the instant of vaporization of the parallel fusible elements, a high resistance formed by the elongated parallel arc columns, which will initially limit the current to a magnitude, which is only a fraction of the current which the short circuit is otherwise capable of producing. The inserted resistance will cause rapid current decay, and the quick cooling of the arcs will produce a subsequent interruption of the current flow in short circuits within the interrupting capacity of the fuse unit.

As background information, reference may be had to US. Pat. No, 3,213,242 issued to Frank L. Cameron. Additional reference may be made to Dewey US. Pat. No. 2,672,540 issued Mar. 16, 1954.

SUMMARY OF THE INVENTION According to a preferred embodiment of the invention, a current-limiting fuse is provided having one or more elongated fusible elements of stepped, or graduated sections of different wire diameter with a plurality of spaced dents provided therealong. This has the advantage of limiting the length of the overall dimensions of the fusible device as compared with prior-art structures,

Accordingly, it s a general object of the present invention to provide an improved current-limiting fuse of reduced axial length.

Still a further object of the present invention is to provide an improved current-limiting fuse which, upon interruption, imposes less electrical stress upon the adjacent insulation than has prior-art current-limiting fuses.

Still a further object of the present invention is the provision of an improved current-limiting fuse of highly effective operation and operable with a minimum of voltage surges upon the line.

Still another object of the present invention is the provision of an improved current-limiting fuse in which the associated dielectric circuit has less voltage stress imposed thereon.

Another object of the present invention is the provision of an improved current-limiting fuse comprising a plurality of BRIEF DESCRII TION OF THE DRAWING FIG. 1 is a side elevational view, partly in section, of a fuse device embodying our invention and illustrating a side view of one of the individual fusible elements;

FIG. 2 is a view of the fusible element of FIG. 1, considered separately from the fuse structure, and illustrating the same from a top plan view thereof;

FIGS. 3 and 3A illustrate the arc voltage and corresponding let-through current waveforms provided by a prior-art single wire fusible device of constant diameter with no dent portions;

FIGS. 4 and 4A illustrate similar voltage and current waveforms obtained by a prior-art dented single wire fusible element with a constant diameter; and t FIGS. 5 and 5A illustrate, respectively, the arc-voltage surges and the let-through fault current waveforms resulting from an application of the novel fusible link of the present invention in a current-limiting fuse.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention relates to a high-voltage currentlimiting fuse for interrupting any fault current flowing through a controlled associated electrical circuit. As well known by those skilled in the art, in general, for use in a high-voltage current-limiting fuse for interrupting fault currents, there has commonly been used a fuse using an arc-extinguishing agent, such as silica sand, or the like, disposed about the fusible element, which is melted with an are energy produced when the fuse element fuses. The resultant arc voltage reduces the fault current, and the arc-extinguishing agent absorbs the arc energy for cooling the resulting metallic vapors, while the fault current is interrupted by the high resistance exhibited by the molten arc-extinguishing agent, such as sand. For the highvoltage current-limiting fuses of the type previously described, the associated fuse element has had considerable length, with corresponding greater interruption length with the effect of producing reliable interrupting operations. However, the disadvantage exists that in such prior-art fuses, the longer the fuse element, the higher the arc voltage developed upon fusing thereof, resulting in a voltage hazard to the insulation of the associated circuit.

In an effort to overcome the above-mentioned disadvantages, the present invention has been conceived to provide a new type of fusible element, which is reliable during the interrupting operation, and low in arc voltage developed upon fusion, as compared with a conventional fuse element commonly used, for example, one formed of a single wire of constant diameter, or one formed of constant diameter with dents therealong.

With reference to the drawing, and with particular attention directed to FIGS. 1 and 2 of the drawing, it will be observed that the reference numeral 1 designates a current-limiting fusible device comprising a plurality of electrically parallel fusible elements 2 composed of a suitable fusible material, such as silver, and electrically connected between the end ferrules 3, 4 of the device 1. As illustrated more particularly in FIG. 1, the end ferrules, or end terminals 3, 4 of the current-limiting fusible device 1 are pressed over the ends of an insulating tubular fuse holder 5, which contains a granular material, such as silica sand 6. Embedded within the sand 6 is a plurality of electrically parallel fusible elements 2, only one of which is illustrated in FIG. 1 of the drawings. It will be observed that the fusible element 2 comprises sections, or portions 2a, 2b, 2c, 2n of different cross-sectional areas. In more detail, the invention resides in a fusible element 2 having different cross-sectional areas d1, d2, dn for predetermined longitudinal lengths L L2, L,, respectively, and having dents 8 on the element of multistepped diameter, as thus formed.

According to the present invention, when a fault current flows through the fusible element 2 to are and fuse the same, the dented portion, or portions 8 of the fusible element 2 with the small cross-sectional area, such as portion 2a, first begin to arc, producing a first portion of the resultant arc voltage. Then, about the time when the one portion of the arc voltage is going to collapse, the element portions having the larger cross-sectional area 2b, etc. there progressively results an arcing of the larger cross-sectional portions 2b etc. until an arc voltage is developed throughout the entire length of the stepped fusible element 2. Thus, as illustrated in FIGS. and 5A, the arc voltages are developed in scattered relationship with respect to time, as illustrated by the reference numeral 10 in FIG. 5 of the drawing. This is to be compared with the use of a conventional type of fusible element in a current-limiting fuse, as formed of a single wire of constant diameter, as illustrated in FIGS. 3 and 3A of the drawing, or a dented fuse element having the same cross-sectional area throughout its entire length, as illustrated by the voltage and current conditions exemplified in FIGS. 4 and 4A of the drawing.

Thus, the arc voltage can decrease for the entire length of the fusible element 2, and this permits the imposition of less voltage surges on the adjacent insulation associated with the device 1. A further advantage is that the interruption length is increased as compared with conventional fuse elements, provided that the associated circuit has the same dielectric strength. Thus, the invention can increase the current-limiting effect and perform more reliable circuit interruption.

From the foregoing description it will be apparent that there has been provided an improved current-limiting fusible device 1 having a plurality of parallel-arranged fusible elements 2, each of which has a stepped, or graduated cross-sectional areas with the sections dented, as at 8, to provide arcs in series. A further desirable result is that due to the progressive arcing, which occurs, the entire fusible device 1 may be made of shorter axial length L.

Although there has been illustrated and described a specific structure, it is to be clearly understood that the same was merely for the purpose of illustration, and that changes and modifications may readily be made therein by those skilled in the art without departing from the spirit and scope of the invention.

We claim:

1. A fuse comprising, in combination, a tubular fuse holder, spaced terminal means supported by said fuse holder, a fuse element electrically connected between said spaced terminal means, said fuse element having different cross-sectional areas of progressively increasin size for predetermined lengths along its axial length and a so being dented at a multiplicity of spaced poitions along its entire axial length, and the smallest cross-sectional predetermined length of said fuse element being immediately adjacent one of said spaced terminal means, and the predetermined lengths of said fuse element being of increasingly larger cross-sectional area to the other spaced terminal means at the other end of the fuse holder.

2. The combination according to claim 1, wherein the fuse element has a round cross-sectional area.

3. The combination according to claim 1, wherein sand fills a portion of the tubular fuse holder.

4. The combination according to claim 1, wherein the fuse element is of silver, and the dents (8) are axially in alignment on the same side of the fuse element.

5. A high-voltage current-limiting fuse, comprising, in combination, a tubular fuse holderat least partially filled with a granular arc-extinguishing material, spaced terminal means supported in spaced relation by said tubular fuse holder, one or more fusible elements having dented portions (8) electrically connected between said spaced terminal means, each fusible element having a plurality of stepped sections (2a, 2b, 2n) of progressively increasing cross-sectional areas, the smallest crosssectional area section (2a) being at one terminal means and connected thereto, and the largest cross-sectional area section (2n) being at the other spaced terminal means and connected thereto.

6. The combination of claim 5, wherein the granular arcextinguishing material is sand.

7. The combination of claim 5, wherein the fusible element is composed of silver. 

1. A fuse comprising, in combination, a tubular fuse holder, spaced terminal means supported by said fuse holder, a fuse element electrically connected between said spaced terminal means, said fuse element having different cross-sectional areas of progressively increasing size for predetermined lengths along its axial length and also being dented at a multiplicity of spaced portions along its entire axial length, and the smallest cross-sectional predetermined length of said fuse element being immediately adjacent one of said spaced terminal means, and the predetermined lengths of said fuse element being of increasingly larger cross-sectional area to the other spaced terminal means at the other end of the fuse holder.
 2. The combination according to claim 1, wherein the fuse element has a round cross-sectional area.
 3. The combination according to claim 1, wherein sand fills a portion of the tubular fuse holder.
 4. The combination according to claim 1, wherein the fuse element is of silver, and the dents (8) are axially in alignment on the same side of the fuse element.
 5. A high-voltage current-limiting fuse, comprising, in combination, a tubular fuse holder at least partially filled with a granular arc-extinguishing material, spaced terminal means supported in spaced relation by said tubular fuse holder, one or more fusible elements having dented portions (8) electrically connected between said spaced terminal means, each fusible element having a plurality of stepped sections (2a, 2b, 2n) of progressively increasing cross-sectional areas, the smallest cross-sectional area section (2a) being at one terminal means and connected thereto, and the largest cross-sectional area section (2n) being at the other spaced terminal means and connected thereto.
 6. The combination of claim 5, wherein the granular arc-extinguishing material is sand.
 7. The combination of claim 5, wherein the fusible element is composed of silver. 